This invention relates to an electrofusion coupler for use in the construction of thermoplastics pipe systems wherein a welded joint between adjoining ends of adjoining sections of plastics pipe, or of such a pipe section and a tubular member forming part of the pipe fitting, valve or the like, is obtained by forcing such adjoining ends into such an electrofusion coupler and energising the same.
Such an electrofusion coupler takes the form of a sleeve of thermoplastics material having an electrical resistance heater embedded therein and extended around the sleeve. When adjoining pipe ends or the like are forced into the sleeve from opposite ends and the resistance heating wire, and thus the sleeve, is subsequently heated by passing an electric current through the wire, both the material of the sleeve and the material of the inserted pipe ends or the like are locally softened and fused, whereby an intimate connection is obtained, in the form of a welded join which, when the assembly has cooled, is leak tight to fluids. The coupler remains in situ forming a permanent part of the joint.
Typically, the electrical resistance wire takes the form of coil embedded in the thermoplastics sleeve and coaxial therewith, the wire lying close to the inner surface of the sleeve, the ends of the wire being connected to terminal points, exposed on the exterior of the sleeve for connection to a source of electrical current.
In the construction of thermoplastics pipe systems using electrofusion couplers of this kind, it is desirable to have some reliable yet inexpensive means for determining whether the interior of the sleeve, and the inserted pipe end or the like have been sufficiently heated to afford a good welded joint or whether it is necessary to re-make the joint.